If you are among the thousands of American women who have survived breast cancer, you probably find yourself thinking twice about everything you do  what you eat, how much you exercise  to ensure that you don't increase your risk of developing another tumor. It's a natural response to a difficult diagnosis, but it can be challenging, especially when it comes to diet: most breast tumors are driven by the hormone estrogen, but estrogen is frequently found in many popular foods, from some types of milk and yogurt to breakfast bars to tofu and those addictive edamame beans.

The common culprit is soy, a plant that contains chemicals with estrogen-like and anti-estrogenic properties  making it a nutritional minefield for breast-cancer survivors. While Western diets are relatively low in soy  compared with the typical diet in Asia, where people eat soy daily  the percentage of Americans consuming soy at least once a week increased from 15% in 1997 to 28% in 2003. In the meantime, studies on the effect of soy on breast-cancer recurrence and mortality have been conflicting, with some showing that it can reduce risk, while others show an elevated rate of recurrent disease among frequent consumers of soy. (See breast cancer's fundraising warrior.)

Now the largest study to date on soy's effect on breast cancer suggests that eating soy, even in large amounts, may not be harmful after all, and may even reduce recurrence and death from the disease. But while the findings are intriguing, not all doctors are ready to tout the benefits of tofu.

Analysis of the study, conducted in China between 2002 and 2006, is ongoing, but researchers based at Vanderbilt University and the Shanghai Institute of Preventive Medicine report data from the first four years of follow-up (total follow-up was five years) in the current issue of the Journal of the American Medical Association. The 5,042 women enrolled in the study were all breast-cancer survivors, ages 20 to 75, and they consumed soy from naturally occurring sources, such as tofu or soybeans; none of the women took soy supplements. They fell into two groups based on soy intake: those who consumed more than 15.3 g of soy protein a day, or as much as would be found in three-quarters of a cup of edamame beans, and those who consumed less than 5.3 g per day, less than what is contained in a half-cup of soy milk, which has 7 g of soy protein. (Read "The Mammogram Melee: How Much Screening Is Best?")

Among the women consuming the most soy, the risk of death from breast cancer four years after diagnosis was 7.4% and the risk of recurrence was 8%. Women in the lower soy-intake group had higher risks: a 10.3% risk of death from breast cancer and an 11.2% risk of recurrence. "I think based on our study, I am quite comfortable saying that soy food, particularly a moderate amount, is safe, and potentially beneficial," says the study's lead author, Dr. Xiao-Ou Shu, an epidemiologist at Vanderbilt.

Other breast-cancer experts agree that soy intake may not be harmful for cancer survivors, but they draw the line at saying it can reduce cancer incidence or mortality. "There is no take-home message here to go out and eat as much soy as you can," says Dr. Larry Norton, medical director of the Evelyn Lauder Breast Center at Memorial Sloan-Kettering Cancer Center in New York City.

Norton notes that the study was not a randomized clinical trial of soy consumption. That is, rather than randomly assigning breast-cancer survivors to consume or not consume various amounts of soy, then following those participants to see whether they developed recurrent tumors, the study looked retrospectively at women's self-determined soy-eating habits. The randomized clinical trial is the gold standard upon which medical practice is determined, and the only kind of trial that gives scientists confidence that other variables are not confounding their results. In the new study, for example, the authors note that the women eating the highest amounts of soy were also more likely to have received chemotherapy to treat their disease and to eat vegetables and exercise regularly  all factors that may contribute to lower breast-cancer risk. Shu and her colleagues adjusted for these variables, but, says Norton, "with any observational study, you don't know what else is going on."

Although not all experts are convinced that it's safe to begin advising women to add soy to their diet, they agree that there is no need to avoid soy altogether. "What I've been telling my patients right now is that soy as part of a healthy balanced diet is safe. But I would avoid trying to eat a totally soy-based diet or taking a soy supplement. You have to be careful in not extrapolating beyond the study," says Dr. Richard Lee, medical director of the Integrative Medicine Program at MD Anderson Cancer Center in Houston.